Systematic exploration of the kinetics of sulfane sulfur transfer by thiosulfate reductase (Ec unassigned) and 3-mercaptopyruvate sulfurtransferase (EC 2.8.1.2) will be carried out. Although the formal mechanisms of these enzymic reactions are quite complex with some substrates, they are simpler with others, and we anticipate with some confidence obtaining data that can be interpreted clearly in terms of chemical mechanism. Recent developments in the study of the sulfurtransferase rhodanese (EC 2.8.1.1) in this laboratory have provided a basis for studies on the relationship of protein-bound sulfane sulfur to protein structure. Somewhat similar studies to this same point will be undertaken by ultrafiltration techniques with the deformable, multidomain, sulfane-containing enzyme xanthine oxidase (EC 1.2.3.2).